A known two-stage torsional elastic coupling disclosed in JP 7-27142 A (Reference 1) is a compression elastic coupling of this type. This known two-stage torsional elastic coupling includes inner elastic members having a low torsional rigidity and disposed near the center of the two-stage torsional elastic coupling and outer elastic members having a high torsional rigidity and disposed in a peripheral part of the two-stage torsional elastic coupling. Power of a low torque is transmitted from a drive flange through the inner elastic members to a driven member. When load on the two-stage torsional elastic coupling is large, power is transmitted from the drive flange through both the inner and the outer elastic members to the driven member.
A known clutch mechanism disclosed in German Patent No. 3432436 (Reference 2) includes dogs and rubber elements interposed between the dogs. Each of the rubber elements includes a pair of parallel metal plates disposed opposite to each other and a rubber member sandwiched between and bonded to the metal plates. Each of the metal plates is welded to one end of a screw. The rubber element is fastened to the dog with a nut screwed on the screw.
The known two-stage torsional elastic coupling and the known clutch mechanism have the following points subject to improvement.
(1) The inner and the outer elastic members of the two-stage torsional elastic coupling mentioned in Reference 1 are solid rubber members and do not include any metal members, such as metal plates. Since the elastic members are not mechanically fastened to another member, gaps are likely to be formed between the input shaft and the output shaft and the connection of the input shaft and the output shaft is likely to be loosened by the plastic deformation of the elastic members due to aging.
(2) As shown in FIG. 13A, the rubber element of the clutch mechanism mentioned in Reference 2 is formed by bonding metal plates 30 to a rubber member 40. Screws are welded to the metal plates 30. The rubber element is interposed between a pair of dogs and the screws are fastened to the pair of dogs to hold the rubber element fixedly between the pair of dogs. Thus any play is not easily formed between the input shaft and the output shaft connected by the clutch mechanism. Although any play is not easily formed between the input shaft and the output shaft because the rubber elements are fixedly held on the dogs, the rubber member 40 of each rubber element is compressed and the rubber member 40 is caused to bulge greatly radially outward as shown in FIG. 13B when power is transmitted through the clutch mechanism. Thus the rubber member 40 is likely to be excessively deformed. The rubber member 40 adhesively bonded to the inside surfaces of the flat metal plates 30 as shown in FIG. 13A is likely to separate from the metal plates 30 when compressively deformed.
(3) In the known two-stage torsional elastic coupling and the known clutch mechanism, the hardness of the material of the elastic members, namely, the rubber members, needs to be changed by changing the material to increase the elastic modulus of the elastic member of rubber. Both the known two-stage torsional elastic coupling and the known clutch mechanism are not provided with any cooling mechanism for cooling the elastic members. Therefore, the elastic members have low fatigue strength.